A three position four-way control valve used for controlling a reversible hydraulic motor typically has a single spool for controlling pump-to-cylinder flow and cylinder-to-tank flow. One of the problems encountered with the use of a single spool is that the timing of the metering spots is designed to optimize the control of the pump-to-cylinder fluid flow. Thus, the spool is generally inadequate for metering cylinder-to-tank fluid flow in an overrunning load condition.
The problem noted above was solved somewhat by the disclosure of U.S. Pat. No. 5,138,838 which uses a pair of electrohydraulic control valves arranged so that each control valve controls fluid flow to and from only one port of a reversible hydraulic cylinder.
In a more recent development, an independent metering valve includes a pair of independently controlled electrohydraulic displacement controlled spool valves controlling pump-to-cylinder communication between an inlet port and a pair of control ports and another pair of independently controlled electrohydraulic displacement controlled spool valves for controlling cylinder-to-tank fluid flow between the control ports and an outlet. Each of the spool valves has a displacement controlled solenoid valve for controlling the position of the spool valve. The spool valves are normally biased to a closed position and are selectively actuated to provide several modes of actuation.
One of the problems encountered with those systems is that while the use of either a pair of electrohydraulic valves or four independently controlled electrohydraulic displacement controlled spool valves can provide many functions normally requiring separate valves simply by actuating one or more of the valves, the functions requiring fast response such as pressure relieving and fluid make up requires the use of special pressure sensors and increased microprocessor computing speed to operate satisfactory.
The present invention is directed to overcoming one or more of the problems as set forth above.